1. Field of the Invention
The invention relates to a single-layered, transparent, biaxially oriented, thermoset tubular food casing, in particular a synthetic sausage casing, based on polyamide.
2. Description of Related Art
Food casings based on polyamide are described in the literature. See, for example, Saechtling, Kunststoff-Taschenbuch [Plastics Pocketbook] or Kunststoff-Handbuch [Plastics Handbook], Volume IV, "Polyamides", or Karmas, Sausage Casing Technology). In many cases, polymer mixtures containing polyamides are used in order to compensate for certain disadvantageous properties of the pure polyamide (see EP-A-0 176 980). For example, although pure polyamide casings can be clear, that is they have a high transparency, their water vapor barrier is substantially lower than in comparable casings made of polyamide blends.
For certain types of sausages, in particular scalding and boiled sausages, it is desirable to maintain the high transparency, aroma tightness and heat and cold resistance of the polyamide casing, while at the same time improving the water vapor barrier and the oxygen tightness. For this purpose, various additives or changes in the composition of the polymer mixtures used or an orientation of the polyamide casings are described in the literature, but as a result of these modifications the transparency of the casing or other important properties are impaired.
Thus it is known, for example, that casings made of polyolefins, in particular polyethylene (PE) and polypropylene (PP), have an outstanding water vapor tightness. However, processing the polyolefin in a blend with polyamides (PA) simultaneously sharply decreases the oxygen barrier of the resulting casing.
Moreover, casings made of such a PA/PE blend have a bluish, mother of pearl-like shine, which gives the impression of a moldy, decayed product. In addition, with an increasing polyolefin fraction in the blend, the sausage meat adhesion becomes progressively poorer and an undesired jelly deposition occurs. Moreover, the printability becomes increasingly poorer with increasing polyolefin fraction.
Further attempts are known from the art to improve the properties of the casings based on polyamide. In particular, in DE-A 39 43 024, a single-layered, tubular, stretch-oriented packaging casing is described which essentially comprises nylon 6 and a part-aromatic copolyamide composed of hexamethylenediamine, terephthalic acid and isophthalic acid units. The sausage casing is clear and has a particularly high resilience, which is said to ensure fold-free fitting of the casing against the sausage. However, in practical use this casing still showed much too high a water vapor permeation, which, particularly with small caliber casings (.phi. &lt;45 mm), leads to undesired weight losses and drying of the sausage. With storage and transport times of the sausages of more than 2 weeks, premature drying of the sausage product takes place in association with a color change, as a result of which the sausage gives an aged, decayed impression.
EP-A 0 065 278 describes a transparent shrinkable film made of at least one layer of a polymer mixture of linear aliphatic polyamide and/or an elastomeric component and 15 to 90% by weight of a part-aromatic polyamide. The elastomeric component described is polybutadiene and mixtures of polyamides with polybutadiene. The film has good shrink properties, good transparency and a high surface polish, the beneficial physical properties of the polyamide being maintained. However, the barrier properties are still in need of improvement.
EP-A 0 358 038 describes a blend of an amorphous nylon copolymer and a nylon copolyamide. The amorphous polyamide component described is a copolyamide composed of hexamethylene units, isophthalamide units and terephthalamide units; the copolyamide component includes copolymers of nylon 6 and a further polyamide, preferably nylon 6/12 or nylon 6/66. Optionally, the blend can contain as an additional component a polyamide homopolymer such as nylon 6, nylon 11, nylon 12. This blend is to be used according to the description for the production of packaging casings and packaging films for foods. In a further embodiment, further layers of suitable polyolefin materials or suitable polyesters can be applied. Such polymer laminates frequently show an unsteady stretching behavior in the production of biaxially oriented tubes. Furthermore, as a result of the multilayer casing design claimed, the production of such a casing is expensive and complicated.
EP-A 0 450 435 describes shrinkable tubular films based on a polyamide/polyolefin polymer mixture which contains aliphatic polyamides and an acid-modified olefin copolymer, in particular ethylene/acrylic acid (EAA) and ethylene/methacrylic acid. This tubular film is said to have an improved barrier effect against water vapor and oxygen and an improved peeling behavior. As a result of the EAA addition alone, the specific O.sub.2 barrier is reduced in comparison with the pure nylon 6 casing. With an increase in the acrylic acid fraction in the copolymer, the O.sub.2 permeation rate can be reduced, but this leads simultaneously to a reduction in the water vapor tightness.